The use of optical forces to trap and manipulate micron size particles was pioneered by A. Ashkin over ten years ago. [See A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm and S. Chu, Opt. Lett., 11,288 (1986)]. He showed that a single, tightly focused laser beam could be used to hold a microscopic particle in three dimensions near the focus of the beam. Apparatus using a beam in this way provides a powerful non-invasive technique for manipulating microscopic particles, and is generally known as “optical tweezers”.
Optical tweezers have firmly established themselves as powerful tools, especially in the field of biology where they have enabled a range of studies to be conducted. This includes work on DNA, colloids, red blood cells, chromosomes and other biological specimens.
Optical tweezers make use of the optical gradient force: for particles of higher refractive index than their surrounding medium, photons from the beam are refracted around the particles and thus impart reaction forces (resulting from their change in momentum) on the particles. The more photons that are refracted in one general direction, the greater the reaction force on the particle in the opposite direction. This results in various particles migrating towards and being held within the region of the beam with the highest light intensity.
However, conventional optical tweezers provide little or no effective control over the orientation of the microscopic particles which they manipulate.
The ability to induce controlled rotation of trapped particles within optical tweezers potentially offers a new degree of control for microscopic objects and has significant applications in optical micro machines and biotechnology. To date, two major schemes have successfully enabled trapped micro objects to be set into rotation. The first scheme employs Laguerre-Gaussian light beams [H. He, M. E. J. Friese, N. R. Heckenberg, H Rubinsztein-Dunlop, Phys. Rev. Lett., 75 826, (1995); M. E. J. Friese, Enger J, H. Rubinsztein-Dunlop, N. R. Heckenberg, Phys. Rev. A54, 1593–1596 (1996); N. B. Simpson, K. Dholakia, L. Allen and M. J. Padgett, Opt. Lett., 22, 52 (1997)]. Such beams possess an on-axis phase singularity characterised by helical wavefronts. Thus, the Poynting vector in such beams follows a corkscrew-like path as the beams propagate, and this gives rise as to what is termed as orbital angular momentum in the light beam [L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, J. P. Woerdman, Phys. Rev. A45, 8185 (1992)].
This angular momentum is distinct from any angular momentum due to the polarisation state of the light and has magnitude of lh where l is one of two indices that describes the mode. Specifically l refers to number of complete cycles of phase (2πl) upon going around the mode circumference. However, to transfer orbital angular momentum to a trapped particle with such a beam, the particle must typically absorb some of the laser light yet still be transparent enough to be tweezed. This in turn restricts the range of particles this method can be applied to and also further limits this technique in that heating from this absorption could damage the rotating particle. Furthermore, as the particle absorption can be difficult to quantify, controlled rotation of trapped objects in such a beam is very difficult to realise.
The other technique for rotation makes use of the change in polarisation state of light upon passage through a birefringent particle [M. E. J. Friese, T. A. Nieminen, N. R. Heckenberg, H. Rubinsztein-Dunlop, Nature, 394,348 (1998)]. For example, circularly polarised light is well known to possess spin angular momentum and this angular momentum can be exchanged with a birefringent medium (e.g. calcite) upon passage of the beam through the medium.
This method has achieved rotation rates of a few hundred hertz for irregular samples of crushed calcite, but it limited solely to birefringent media, is difficult to control and is thus not widely applicable. Thus both these methods have serious shortcoming for rotating optical microcomponents.